i965: Implement ARB_compute_variable_group_size

i965: Implement ARB_compute_variable_group_size

This patch adds the implementation of ARB_compute_variable_group_size
for i965. We do this by storing the local group size in a push constant.

Additional changes made by Caio Marcelo de Oliveira Filho.

Signed-off-by: Plamena Manolova <plamena.manolova@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4504>